Here - PMOD/WRC

Figure 1. Then new NPL cavity pyroelectric detector.
A total of 35 CPDs have been assembled and studied over
the period from 1998 to 2003. The gain G(λ) [5] as a
function of wavelength was measured for the majority of
these CPDs. Figure 2 shows the spectral responsivity of a
3 mm by 3 mm Pt-black-coated pyroelectric detector
measured in the 0.8 µm to 25 µm wavelength range
relative to both the CPD75 and pyroelectric detector 75 i.e.
against the same pyroelectric detector, with and without a
gold-coated reflective hemisphere. These plots are typical
for the pyroelectric detectors with the very best quality
gold-black coatings. Division of the two traces shown in
Figure 2 gives the relative spectral responsivity of CPD75
relative to pyroelectric detector 75. This ratio is also the
gain G(λ) resulting from the addition of the reflective
hemisphere onto the pyroelectric detector. The values of
G(λ) as a function of wavelength for CPD75 are shown in
Figure 3. Figure 3 shows that the gain is greater than unity
for all wavelengths. This confirms that the gold-black
coating deposited on pyroelectric detector 75 is not perfect
and that the addition of the hemisphere redirects some of
the radiation reflected by the gold-black coating back to
the gold-black coating thus enhancing its effective
absorbance and therefore its response. Moreover Figure 3
shows that G(λ) increases monotonically with increasing
wavelength. This indicates that the reflectance of the gold
black coating gradually increases with wavelength
(assuming the effective reflectance of the reflecting
hemisphere is not a function of wavelength). This
observation is in agreement with measurements of the
reflectance of all good quality gold-black coatings [6].
Relative response
2
1.9
1.8
1.7
1.6
1.5
1.4
T13/cp75
T13/P75
1.3
0 5 10 15 20 25
Wavelength/µm
Figure 2: Spectral response of a 3 mm by 3 mm
Pt-black-coated pyroelectric detector measured in the
0.8 µm to 25 µm wavelength range relative to both CPD75
and pyroelectric detector 75
All parameters of the cavity pyroelectric detectors were
fully evaluated and will be presented at the meeting. These
include their relative and absolute spectral responsivities,
spatial uniformity of response, linearity of response,
temperature coefficient of response, temporal response,
long term stability and noise characteristics.
The presentation will also describe how the relative
spectral responsivity of a cavity pyroelectric detector is
estimated from the knowledge of its gain G(λ) as a
function of wavelength. It will also deal with the
uncertainty budget developed to estimate the uncertainty of
the NPL infrared relative spectral responsivity scale.
Gain
1.045
1.04
1.035
1.03
1.025
1.02
1.015
1.01
1.005
1
0 5 10 15 20 25
Wavelength/µm
Figure 3: Gain in the 0.8 µm to 25 µm wavelength range
measured when a reflective hemisphere was added to
pyroelectric detector 75.
Acknowledgements Special thanks must go to John Lehman of
NIST Boulder for assembling all the pyroelectric detector
modules used in the new NPL CPDs and for many useful
discussions. The author also wishes to thank Nick Nelms of the
Space Research Centre of the University of Leicester, UK and
Mark Proulx of Laser Probe Inc.
Reference
1. G. W. Day, C. A. Hamilton and K. W. Pyatt,
Applied Optics, 15, 1865-1868, 1976.
2. D. H. Nettleton, T. R. Prior and T. H. Ward,
“ Improved spectral responsivity scales at NPL, 400
nm to 20 µm”, Metrologia, 30, 425-432, 1993.
3. L. P. Boivin, “Current work at NRC of Canada on
absolute radiometer based calibrations in the infrared”,
in New developments and Applications in optical
Radiometry, N. P. Fox and D. H. Nettleton (eds), 1989,
81-88.
4. J. Lehman, E. Theocharous, G. Eppeldauer, and C.
Pannel, “Gold-black coatings for freestanding
pyroelectric detectors” Measurement Science and
Technology, 14, 916-922, 2003.
5. E. Theocharous, “The establishment of the NPL
infrared relative spectral responsivity scale using
cavity pyroelectric detectors”, to be published.
6. N. Nelms and J. Dawson, “Goldblack coating for
thermal infrared detectors” Accepted for publication in
Sensors and Actuators, January 2005.
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